The embodiments of the present invention relate generally to systems for handling blow out preventer (BOP) stacks. More particularly, the embodiments provide a system for transporting and handling a BOP stack during installation and removal from a wellhead.
Rigs used for drilling hydrocarbon wells are large, complex pieces of machinery. While drilling rigs used offshore are often integrated into a single, large platform, almost all rigs used to drill wells on land are designed to be disassembled, transported between drilling sites, and reassembled. Although some rigs may be designed to be moved by helicopter or airplane, the majority of rigs are moved by trucks and trailers. Thus, many land rigs are designed to disassemble into components suitable for transport.
The process of assembling a land rig for drilling operations is known as “rig up.” During rig up, all of the various components of the drilling rig are assembled and tested prior to any drilling activity taking place. The rig up procedure may last anywhere from a couple of days to more than a week, depending on the type of rig being assembled and any problems encountered during the process. Because, drilling the well can not commence until rig up is complete it is desirable to minimize the time spent assembling the drilling rig.
The entire rig up process must be performed in reverse in order to disassemble, or “rig down,” the rig for transportation to another location. The rig down procedures further add to the downtime that the rig spends between drilling wells. The amount of downtime spent between drilling wells is often limited by the contracts under which the rigs are operated such that any time beyond a certain limit will not be paid for by the rig lessee. Thus, any equipment or procedures available, which limit the amount of time needed for rig up and rig down activities, are desirable.
One of the most time consuming and labor intensive tasks during rig up and rig down is the handling of the blow out preventer (BOP) stack. BOP's are essentially large diameter, high pressure valves used to control flow out of the wellbore, a BOP stack often includes several individual BOP's assembled in series. In oilfield vernacular, the terms BOP, BOP stack, and stack are all used in referring to the BOP stack. The BOP is installed at the wellhead (beneath the drill floor) and all equipment and fluids traveling into or out of the well during drilling pass through the BOP. The BOP is the last line of defense in preventing the uncontrolled release of wellbore fluids at the surface, known as a blowout, and are therefore a critical piece of safety equipment on the rig. On large land rigs, the BOP may have a 13″ or greater bore diameter and be rated for working pressures up to and exceeding 10,000 psi.
In normal operations, several individual BOP's are stacked on top of one another to form a “BOP stack.” Typical stacks are tens of feet high and weigh in at tens of thousands of pounds. On most land rigs the stack is at least partially disassembled during transport because the rig has no practical means of transporting the fully assembled stack. The heavy-duty connections between individual BOP's within a stack often take hours to make or break, adding to the time needed for rig up or down.
Once the BOP is assembled, it must be positioned under the rig floor directly over the wellhead. This is often a delicate, time consuming operation because the large, heavy BOP stack must be moved underneath the already erected rig. The BOP must be centered on the axis of the well, which runs from the rotary table on the drill floor into the wellhead, thus potentially requiring position adjustment in two directions on the horizontal plane. The BOP stack must also be able to adjust vertically to compensate for differences in elevation of the wellhead. Furthermore, because the BOP stack normally attaches to the wellhead by a flange, which has a bolt pattern that must align with a corresponding bolt pattern on the BOP, the BOP must be allowed to rotate about its vertical axis in order to find the correct alignment with the wellhead.
Most BOP handling systems and methods currently being used involve transferring the BOP stack from one piece of equipment to another, such as from a skid to an overhead lifting system. Many of these overhead lifting systems, such as cranes or trolleys, involve lifting and suspending the BOP, which, like lifting any large load, consumes significant amounts of time and resources to perform safely.
Thus, there remains a need in the art for systems to increase the efficiency and safety of handling a BOP stack during rig up and rig down procedures. Therefore, the embodiments of the present invention are directed to methods and apparatus for providing for a BOP handling system that seeks to overcome the limitations of the prior art.